Optoelectronic devices such as photovoltaic devices are often made in the form of one or more cells. Each cell typically includes an active layer (sometimes including a window layer) between two electrodes, at least one of which is usually transparent. The active layer of the cells absorbs light to produce a current and voltage. Thin-film inorganic cells for optoelectronic devices are often contain polycrystalline inorganic materials in the active layer. Polycrystalline materials have distinct grains of crystal structures separated by grain boundaries. Unfortunately devices made from such cells can suffer from losses in efficiency, stability, and reproducibility due to effects at the grain boundaries arising from the polycrystallinity of the active layer of the cell.
The active layer of inorganic cells can be comprised of a range of light-absorbing and charge-transmitting materials. For example, an active layer can be created by forming an alloy of copper, indium, gallium, and selenide or sulfur (CIGS). Alternatively, an active layer can be created by synthesizing C admium Telluride (CdTe) or C admium Selenide (CdSe). Copper Sulfide can also be used an inorganic absorber material in the active layer. Additional inorganic active layers include blends, alloys, and mixtures of metals from groups IB, IIIA and VIA in the periodic table of elements. For each of these inorganic materials, the absorber layer typically forms in crystalline domains.
While the ideal target of an inorganic solar cell (CIGS, CdTe, etc.) may be one that is (or closely approximates) a single-crystalline cell, it is generally most economical and commercially viable to apply techniques and processes that produce cells with a poly-crystalline active absorber layer. FIG. 1 illustrates a solar cell device 100 according to the prior art. The device 100 can be built up from a substrate 102 and includes bottom electrode 104, a window layer 106, e.g., cadmium sulfide (CdS), an active layer 108 e.g., Copper Indium Gallium Selenium (CIGS), and a top electrode 110. The window layer 106 and active layer 108 often tend to form distinct grains separated by grain boundaries 107, 109 respectively. The grain boundaries 107, 109 inhibit carrier transport, provide sites for carrier non-productive recombination, and facilitate unwanted diffusion of materials (e.g. the diffusion of S from CdS in inorganic solar cells—a known cause of poor long-term stability). The grain boundaries 107, 109 also tend to be irregular and varied in size, which can give rise to issues of variability and/or reproducibility arising from the uniform/non-uniform application of inorganic materials, and correspondingly varied active layer performance in different areas of the cell. All of these effects tend to degrade to performance of the solar cell 100.
Further, certain active layer materials such as grains comprised of combinations of copper with indium and/or gallium and selenium or sulfur tend to have poor adhesion to common substrate materials. In the prior art, the substrate 102 has been coated with about 500 nm of (relatively expensive) molybdenum (which may double as the electrode 104) to promote adhesion between the CIGS layer and that underlying substrate 102.
Thus, there is a need in the art, for an optoelectronic cell architecture that addresses the detrimental effects of grain boundaries and a corresponding method for making such a cell.